Management of breathlessness is a central challenge in the care of patients with life-limiting illness. Unlike other symptoms, breathlessness worsens at the end of life; the magnitude of distress it causes is on par with the distress and disability caused by pain and fatigue. The role of oxygen therapy in this setting remains controversial despite its widespread use. Oxygen clearly benefits people with severe hypoxemia (PaO2 less than or equal to 5mmHg). When PaO2 is greater than 55mmHg and other funding criteria for home oxygen are not met, palliative oxygen is commonly provided on a compassionate basis for people with intractable breathlessness. In these cases the goal is relief of the sensation of breathlessness and improvement in quality of life, yet we do not know how effective or burdensome such treatment is. We are poised to conduct the definitive study of the role of palliative oxygen for the relief of breathlessness conducted by a multi-disciplinary group of international leaders in the field who have proven their commitment to the project and ability to work together through pilot feasibility studies and previous collaborative efforts. This study is an international multi-center randomized double-blind controlled trial of oxygen versus medical air in the palliative care setting. The specific aims are to test the effectiveness of oxygen versus air in relieving the sensation of breathlessness and improving quality of life, to identify predictors of effect and burdens of the interventions, and to analyze costs. Eligible participants have intractable breathlessness at rest or with minimal exertion and PaO2>55mmHg; 240 participants will be recruited from seven sites on three continents. Oxygen or medical air will be delivered through identical appearing concentrators and nasal cannulae for seven days; participants and investigators will be blinded to the gas provided. Outcome measures reflect the specific aims and the primary outcome is breathlessness on a 0-10 numerical rating scale. Feasibility of recruitment and trial execution have been piloted. The case report forms has been finalized. A secure web-based data management system have been designed and piloted. All standard operating procedures are in place. This study is feasible and ready to be initiated. Breathlessness at the end of life is distressing to patients and caregivers. The results of this study will significantly advance our knowledge of how to better care for this large group of distressed individuals. The results will also advance a long-standing international debate about the role of palliative oxygen for intractable dyspnea and it's funding. [unreadable] [unreadable] [unreadable]